A Low Cost Automatic Irrigation Controller Driven by Soil Moisture Sensors

A substantial percentage of farmers in Southern Africa, including Zimbabwe, rely on rain for most of their farming activities but there are areas which are entirely dependent on irrigation water to produce viable crops. Water-saving agricultural practices and sound water management strategies are therefore required to ensure viability of the farming industry in those places that receive less rain. The study aimed at developing an automatic irrigation controller which is low cost and reliable for a low income farmer. The controller uses signals from the soil to schedule irrigation and was made from cheap and off the shelf components from our laboratory stores and local electronic retail shops. The heart of the controller circuit was the PIC Microcontroller 16F872 that uses only 35 instructions for programming in assembly language. Two dielectric capacitance sensors (0.20 m ECH2O probe, Decagon Devices, Inc. Pullman, WA USA.) connected to the controller circuit, were used to measure the dielectric constant of the soil in order to determine its volumetric water content and hence the need to irrigate. Most of the low cost irrigation controllers that are locally available are ON/OFF type and these cannot give optimal results in terms of irrigation costs and crop yield. We determined that using our controller we could produce compatible results with other watermark methods for scheduling irrigation at lower costs (The total cost of other materials excluding the landed cost of the capacitance sensors and labour for populating the pc board was US$36). We also managed to provide calibration data for soil water based irrigation control in the clay-loam soils of Northern Harare using the capacitance sensors.